This invention relates to the manufacture of a plastic container which may be used for the shipment, storage and sterilization of items such as biological specimens, medical instruments, hospital linen and foods. The plastic container in most instances is in the form of a bag or a pouch having two generally parallel walls which are joined together at their respectively mated side and bottom edges to form a support for any item held within the container. The upper edges of the two walls are matched but are freely spaced from one another to define a container mouth for access to the support portion of the container. The container mouth may be provided with any one of the many types of known openable and reclosable, interlocking fasteners which are capable of being fitted integrally with the upper wall edges defining the container mouth. In general, such fasteners extend entirely across the container mouth, contacting the two sealed edges at the sides of the container to form junctions therewith. When the fastener is in a closed position, it provides a relatively good seal for the container, and as a result such containers have found wide commercial acceptance. However, it has been found that the seal provided by interlocking fastener closures are not entirely satisfactory since they have a tendency to leak, and thus their use in some applications has been limited.
Typically, the two-wall plastic containers which are fitted with the interlocking fasteners of the prior art are made in the following manner. A sheet of plastic stock is folded longitudinally to form a two-wall structure, one wall being connected to the other at this stage by only a folded bottom. The side edges of the walls in the folded structure are aligned by the folding step and the top edges of the walls are spaced and offset to accommodate the subsequent placement of an interlocking fastener along these top edges. Since the interlocking fasteners of the prior art usually have two mating portions, one portion of the fastener is sealed to the top edge of one container wall while the other portion of the fastener is sealed independently to the top edge of the other wall. The two top edges now each having a portion of the fastener sealed thereto are aligned and pressed together to interlock the fastener portions which, in turn, serves to interlock the two top edges. At this stage of the process, the container takes the form of a generally tubular structure having a bottom closed by a fold, two top edges interlocked together by the fastener, and two side edges which are still in an aligned, but freely spaced, relationship. In order to form a closed container, heat and pressure are applied to each of the side edges to heat seal them together along their entire length. Accordingly, the prior art containers are formed in the shape of a bag or a pouch having two heat-sealed side edges, a folded bottom and a mouth portion which is openable and reclosable by means of an interlocking fastener secured to the top edges of the container.
During the heat sealing step of the prior art process, pressure was normally applied along the entire length of the side edges of the container being formed, and included the application of pressure at the junction of each side edge with the interlocking fastener that extended across the top edges of the container. The application of pressure at these junctions was considered necessary to effectively heat seal the side edges to each other and to the top edges of the container walls as well as to close the ends of the fastener. Unfortunately, it was found that containers made in this manner tend to leak and thus were unsatisfactory for the storage or transport of fluid materials.
Upon an examination of the prior art containers to determine why they were incapable of holding fluids, it was found that separation flaws had formed in the fastener near the junction of the fastener with the side edges of the container. It has now been discovered that there are two basic causes of these separation flaws. First, the application of pressure during the heat-sealing of the junction frequently places excessive stress on portions of the fastener at or immediately adjacent the junction, causing the interlocking parts of the fastener to spread or move, thus distorting the fastener profile. Second, the bulk and shape of the fastener often prevents the sealing mechanism from holding the parts of the container in firm contact after the plastic has been heated, so that a strong, continuous weld is not formed. These two factors permit, or even cause, a small separation of the container walls and the interlocking fastener at the edge junction, and a separation of the fastener itself, and these separations have been found to remain as flaws in the integrity of the fastener after the heat sealing step is terminated. Accordingly, even though the finished containers of the prior art are substantially fluid tight because of the folded bottom and heat-sealed side edge construction, they are often found to have a separation flaw at one or both ends of the fastener, at or immediately adjacent the junction of the fastener with the side edge portions of the container. When fluid is placed in these flawed containers and the interlocking fastener is closed for shipment or storage, the fluid has had a tendency to seep into the fastener and travel therealong until it reaches the separation flaws at the ends of the fastener, where the liquid seeps through the separation to the exterior of the container. Further, in the case where objects in the container were originally in a sterile condition, it was found that contaminating bacteria could follow the same path, and thus pass through the separation flaw and into the container.